Referring to FIG. 1, a LED driver, including a power converter 10, a current regulator 12, and a compensator 14, provides an output voltage Vout and a load current Iload to drive a plurality of LED strings 16. The power converter 10 converts an input voltage Vin into the output voltage Vout, and the current regulator 12 controls the current of each LED string 16 to thereby control the brightness of the LEDs. The current regulator 12 includes a plurality of control terminals VS1-VSN, each of which is connected to a respective one of the LED strings 16, and a plurality of transistors M1-MN as switches connected to the control terminals VS1-VSN respectively and controlled by a PWM signal Spwmi. The current, and consequently the brightness, of each LED string 16 can be adjusted by adjusting the duty ratio of the PWM signal Spwmi. The compensator 14 is connected to a feedback path of the power converter 10 to filter out noise in the feedback loop and to stabilize a compensation voltage Vcomp. FIG. 2 is a waveform diagram of the signals shown in FIG. 1. When the PWM signal Spwmi is triggered or terminated, the gate voltages VG1-VGN of the transistors M1-MN are simultaneously turned to a high level or a low level to simultaneously turn on or off the transistors M1-MN. As a result, all the LED strings 16 are activated or deactivated at a same time, and thus the load current Iload provided by the power converter 10 rises or falls abruptly, causing ripples 18 in the output voltage Vout of the power converter 10. The output capacitor Cout will be charged or discharged by the ripples 18 and thus generates audio noise.
FIG. 3 depicts one conventional method for reducing the audio noise, which employs phase delay technique in the current regulator 12 to apply different delay to the LED strings 16. Thus, when the PWM signal Spwmi is triggered or terminated, the LED strings 16 are activated or deactivated at different times, and thereby the load current Iload increases or decreases step by step. This avoids an instant large change in the load current Iload, and thereby reduces the amplitude of the ripples 18 in the output voltage Vout to alleviate the audio noise problem.